Er - it's the stuff that makes whitewash white Les. Whitewash is a mixture of lime, whiting and size. You used to be able to get it from any corner ironmongers but now you probably have to go to a builders' supply firm or maybe an artists' colourman.
You could probably substitute the much more expensive gesso powder. It gives modelling consistency papié maché a lovely surface, but is no good for Punch heads since it isn't resistant to banging and does reduce the strength of the paper. It also adds weight to the head.